The present invention relates to the use of a separator to partition a particulate assemblage into various constituents based on a difference in particle mass and more particularly to partition a particulate assemblage into various constituents based on a difference in particle mass after the specific gravity of one or more of the components in the assemblage has been decreased by the selective attachment of air bubbles.
Gravity concentration devices are used extensively throughout the minerals industry to concentrate high-density particles from a mixture of high- and low-density material. Although many devices have been developed over the years, a technique gaining in popularity is hindered/fluidized-bed separators. These separators, traditionally used for classification, work reasonably well for mineral concentration if the particle size range and density difference are within acceptable limits.
A great deal of research has been devoted to the study of fluidized-beds and their use in gas/solid contacting and in liquid/solid applications. Studies describing the latter have typically focused on the classification aspects of fluidized-bed separators and less so on mineral concentration. Recent work has shown that fluidized-bed separators can be used to effectively separate mineral assemblages that have components with different densities. For instance, coal and the ash forming components (rock), silica and iron ore, and silica from various heavy minerals such as zircon and ilmenite. Results from these studies indicate that efficient concentration can be achieved if the particle size ration (top size to bottom size) is less than 3 or 4 to 1 and in a range from 200 mesh to several millimeters. Unfortunately, this is seldom the case and, as a result, the separation efficiency is poor. To correct this shortcoming, the valuable component (i.e., coal, iron ore, ilmenite and zircon) frequently must be reprocessed to achieve the desired quality.
A hindered-bed separator is a vessel in which water is evenly introduced across the base of the separator and rises upward. The separator typically has an aspect ratio of two or more and is equipped with a means of discharging solids through the bottom of the unit. Rising water and solids flow over the top of the separator and are collected in a launder. Solids are typically introduced in the upper portion of the vessel and begin to settle at a rate defined by the particle size and density. The coarse, higher density particles settle against the rising flow of water and build a bed of teetering solids. This bed of high-density solids has an apparent density much higher than the teetering fluid (water). Since particle settling velocity is driven by the density difference between the solid and liquid phase, the settling velocity of the particles is reduced by the increase in apparent density of the teetering bed. As a result, the low-density component of the feed resists penetrating the bed and remains in the upper portion of the separator where it is transported to the overflow launder by the rising teeter water.
Coarse, low-density particles, however, tend to gather at the interface between the high and low density particles because the teeter water velocity is not sufficient to transport this material to the overflow launder. These particles continue to gather at the bed interface and eventually migrate into the teeter bed, thus reporting with the high-density product. This inherent inefficiency can be partially corrected by increasing the teeter water velocity to convey the coarse, low-density solids to the overflow. Unfortunately, this approach will also cause the fine, high-density solids to be misplaced to the overflow launder resulting in a loss of efficiency. It can be seen, therefore, that a conventional hinder-bed separator has inherent inefficiencies when treating a mineral assemblage that has a Wide particle size distribution and/or a narrow density distribution.
Applicant is aware of the following U.S. Pat. No. 2,758,714 to Hollingsworth; U.S. Pat. No. 4,396,396 to Mainwaring; U.S. Pat. No. 4,822,493 to Barbery; U.S. Pat. No. 5,307,937 to Hutwelker and U.S. Pat. No. 5,456,362 to Laskowski.
From the discussion presented above it is apparent that modifications should be incorporated into new devices to correct the inefficiencies associated with conventional hindered-bed separators. Typically, the particle size and density distribution of the feed cannot be modified. Therefore, a different approach must be considered. The most obvious means is to further enhance the density difference between the low- and high-density particles in the feed. Similar approaches have been evaluated such as introducing a second lower density liquid that has an affinity for a particular species of particles. A subsequent liquid phase separation is used to concentrate the solids. The use of this oil agglomeration technique has been successfully demonstrated for the separation of coal and ash forming impurities. Unfortunately, this technique suffers from high operating costs and low process capacities.
A distinctive feature of the present invention is aeration of a hindered-bed of solids to modify the effective density of one or more of the species. Aeration is achieved by introducing fine air bubbles with the teeter water supply. The bubbles rise with the upward current, impinge upon the particles and selectively attach to the surface of a particular species. Attachment depends upon the surface characteristics of the particle. For instance, coal is naturally hydrophobic and will spontaneously attach to an air bubble. Applications such as iron ore (with a silica contaminant) require chemical activation of the silica to promote bubble/particle attachment. The method for chemical activation is well known and is routinely used for flotation of fine particles (less than 0.2-0.3 mm).
The concept of bubble-particle attachment in a rising current separator (flotation column) has been previously demonstrated. Unfortunately, the approach uses an open-column reactor operating in the free, not hindered, settling regime. As a result, this configuration does not have the advantages associated with a hindered-bed separator. The distinctive advantage of the present invention is the synergy offered by the combination of a hindered-bed separator and a rising current, open-column flotation cell. The approach combines the pre-concentration of a hindered-bed with the further enhancement of bubble-particle attachment to modify the density of one of the feed components. As a result, separation of coarse, low-density material is greatly enhanced through the addition and attachment of air bubbles. Furthermore, since any particulate species can be chemically rendered hydrophobic, applications are not limited to materials having different densities. In fact, in some instances, the high specific gravity mineral can be enhanced through air addition and will report to the overflow as the light species. The efficiency of the new separator surpasses that which can be achieved by either individual technique.
To recognize the advantages of the invention the fundamental difference between free and hindered-settling conditions must be examined. Separators are generally recognized as falling into one of two categories: free settling or hindered settling. Under free settling conditions individual particles do not affect the settling behavior of adjacent particles and, as such, the pulp has the Theological characteristic of the fluid. Furthermore, the settling velocity is determined by particle size and density as dictated by Stokes"" law. Under such conditions, less selective concentration of minerals is achieved.
Hindered settling is fundamentally different. At high solids concentrations, adjacent particles collide with each other influencing the settling characteristics. The settling path is greatly obstructed reducing particle velocity. Additionally, the high solids concentration increases the apparent viscosity and specific gravity of the pulp, thus further reducing particle settling. As a result, the acceleration of particles becomes more important than the terminal velocity. This collision phenomenon is the most important aspect of hindered settling and provides a greater degree of mineral concentration than can be achieved in a free-settling environment.
In addition to the synergy of the combined process, the bubble-particle attachment aspect is also greatly enhanced as a result of the hindered bed. In mineral flotation, it is well known that the recovery of a particular species is predominantly controlled by two parameters: reaction rate and retention time.
An increase in either parameter provides a corresponding increase in recovery. A distinct advantage of this invention is an increase in both reaction rate and retention time beyond that which can be obtained by existing devices.
The reaction rate for a process is indicative of the speed at which the separation will proceed. The rate of separation in a hindered bed is most strongly influenced by the specific gravity difference between particulates. Modifying the particle density through bubble attachment decreases the apparent particle density thus increasing the rate at which material is extracted from the system.
Another advantage of the invention is the increase in the probability of collision between a bubble and particle. It is well known in mineral flotation that the reaction rate is controlled by several probabilities: collision, attachment and detachment. The probability of attachment and detachment are controlled by the process surface chemistry and cell hydrodynamics, respectively. Collision probability, however, is directly proportional to the concentration of particles in the cell. In an open (free settling) system, the collision probability is quite low due to the low particle concentration. A hindered bed provides the highest possible particle concentration while still maintaining the material in a fluid environment; thus maximizing the bubble/particle contact frequency. As a result, the reaction rate is several orders of magnitude higher than that demonstrated by existing systems.
The retention time is a direct measure of the length of time which material is present in the separator. The longer the retention time, the higher the probability the material will be influenced by the reaction. For example, a particle in a flotation cell may not initially encounter a bubble. However, the likelihood of this encounter increases with the time spent in the cell. In the flotation process, particles settle vertically through the cell either with the fluid flow (co-current) or opposite to it (counter-current). A counter-current arrangement has obvious advantages since the settling velocity is reduced by the upward flow of liquid resulting in a higher retention time. Hindered settling, as previously explained, provides an environment in which the particles never achieve their terminal free-fall velocity. As a result, the effective particle velocity through the cell is greatly reduced providing a significant increase in retention time as compared to a free-settling system.
Based on the above description, it is obvious that the separation is not limited to materials with vastly different specific gravities since the density can be modified through the addition of air bubbles.
It is an object of the present invention to provide a separator, which uses air to create or increase the difference between the effective or apparent specific gravities of the materials to be separated.
It is another object of the present invention to provide an air-assisted density separator device that is simple in construction, economical to manufacture and simple and efficient to use.
With the above and other objects in view, the present invention consists of the combination and arrangement of parts hereinafter more fully described, illustrated in the accompanying drawing and more particularly pointed out in the appended claims, it being understood that changes may be made in the form, size, proportions and minor details of construction without departing from the spirit or sacrificing any of the advantages of the invention.